The invention relates to electromagnetic coils. The invention relates more particularly to electromagnetic coils in which the coil is formed of a flat metal and dielectric strip wound onto an inner hub, and to methods and apparatus for making such coils.
Coils of the above-described type are commonly used in high-energy impact devices such as electromagnetic riveters (EMR), dent removal equipment, and the like. Exemplary coil constructions and manufacturing methods for such coils are described in U.S. Pat. Nos. 4,151,640 and 4,146,858, the entire disclosures of which are incorporated herein by reference. The coil disclosed in these patents has an end portion of the strip at the innermost turn of the coil bent 180xc2x0 about an axis lying in the plane of the strip and extending diagonally across the width of the strip such that the strip is folded upon itself, thereby creating a terminal lead for the coil that exits the coil in an axial direction; likewise, the end portion of the strip at the outermost turn of the coil is bent in the same fashion to create an outer terminal lead for the coil extending in the axial direction. The inner and outer terminal leads are then bent a second time through 90xc2x0 bends to direct them radially outwardly from the coil for attachment to electrical conductors that extend away from the coil in the radial direction. The fold regions of the strip represent potential locations for fatigue failure because of the electromagnetic forces and the mechanical stress risers that are generated at the folds.
Another drawback of the above-mentioned coil arises when the coil is a moving coil, such as when used as the moving coil of a dual-coil EMR gun. In this type of application, high accelerations are imposed on the coil along the central longitudinal axis of the coil, and hence large stresses tend to be exerted on the electrical conductors. Unless the connections between the coil leads and the electrical conductors are carefully designed and the connections and conductors are properly secured, which is typically not the case, these stresses can cause the connections between the conductors and the coil terminal leads to loosen over time, resulting in arcing and possible coil terminal damage.
The present invention seeks to overcome the drawbacks noted above by providing a coil assembly, and methods and apparatus for making a coil assembly, in which the terminal leads are formed without folding the coil strip, thereby eliminating or at least greatly reducing the potential for fatigue failure of the strip, and in which the connections between the terminal leads and the electrical conductors are secured in a fashion that substantially reduces the stresses imposed on the conductors and connections.
To these ends, an electromagnetic coil assembly in accordance with one preferred embodiment of the invention includes an inner hub having a central longitudinal axis, and a ring-shaped outer body mounted about the inner hub generally concentric with the longitudinal axis. The coil assembly further includes a coil wound about the inner hub. The coil comprises a flat strip having opposite planar surfaces, the strip being wound about the inner hub in a spiral fashion. The strip generally comprises a flat strip of conductive metal such as copper, and a dielectric material covering at least one of the planar surfaces of the metal strip. In a preferred embodiment, the dielectric material comprises a dielectric fiber material such as fiberglass that is impregnatable or pre-impregnated with a curable binder. The coil has an inner coil lead comprising an inner end portion of the flat strip extending generally axially away from the coil and being joined integrally with the inner turn of the coil at an inner bend region of the strip. The coil also has an outer coil lead comprising an outer end portion of the flat strip extending generally axially away from the coil and being joined integrally with the outer turn of the coil at an outer bend region of the strip. The inner and outer bend regions of the strip are characterized by an absence of folding of the strip onto itself.
More particularly, it is preferred to form the bend regions as flat bends of about 90xc2x0. That is, the strip at its inner and outer ends is bent through an angle of substantially 90xc2x0 about a bend axis that is substantially perpendicular to the opposite planar surfaces of the strip, thereby forming terminal leads that extend in the axial direction parallel to the central longitudinal axis of the coil. In this way, folding of the strip onto itself is avoided.
To substantially reduce mechanical stress on the coil terminal leads and the connections to the electrical conductors, the end faces of the inner hub and outer body have recesses formed in them. The inner terminal lead extends into a recess in the inner hub and is connected to an electrical conductor within the recess by a terminal connector. The outer terminal lead similarly extends into a recess in the outer body and is connected to an electrical conductor within the recess by a terminal connector. The terminal connectors are secured within the recesses. Preferably, the terminal connectors and electrical conductors are secured by potting them in place in the recesses with an elastomeric potting compound, and then a substantially rigid plate is attached across the end faces of the inner hub and outer body so as to cover the potted-in terminal connectors and conductors. This arrangement substantially reduces the stresses on the coil leads and electrical conductors. The elastomeric potting compound preferably allows the electrical conductors to have a slight amount of xe2x80x9cgivexe2x80x9d when accelerations are imposed on the coil assembly.
An apparatus for making a coil assembly in accordance with the invention includes a bending device operable to bend a flat coil strip about a bend axis substantially perpendicular to opposite planar surfaces of the strip to form a substantially 90xc2x0 flat bend in the strip, and a winding device structured and arranged to grip a generally cylindrical inner hub of the coil assembly with the strip secured thereto, and to rotate the inner hub about a central longitudinal axis of the inner hub to cause the strip to be wound about the inner hub.
In a preferred embodiment, the bending device comprises a clamp for clamping the strip in a fixed position with an end portion of the strip projecting out therefrom, and a rotatable bending assembly operable to bend the end portion of the strip to form a circular-arc bend in the strip. The rotatable bending assembly preferably comprises a pair of rollers each defining a strip-receiving slot extending circumferentially therearound and arranged on opposite side edges of the strip such that each side edge of the strip is received within one of the slots. The rollers are mounted on a lever that is rotatable relative to the clamp about a rotation axis substantially perpendicular to the planar surfaces of the strip mounted in the clamp, such that rotation of the lever about the rotation axis causes the strip to be bent.
A coil assembly is manufactured in the apparatus by forming a 90xc2x0 flat bend at one end of the coil strip to form the inner coil lead. This end of the strip is then secured to the inner hub mounted in the winding device, and the winding device is operated to rotate the inner hub to wind the strip onto the hub under tension. The opposite end of the strip is processed in the bending device to form a 90xc2x0 flat bend to create the outer coil lead, preferably in such a manner that both the inner and outer coil leads exit the coil in the same direction parallel to the central longitudinal axis of the coil. The winding of the strip onto the inner hub is then completed and the strip is clamped to the hub to prevent unwinding. The outer body is then mounted to the assembly of the inner hub and coil.
The coil assembly is then clamped between a pair of tooling plates disposed against the opposite end faces of the coil assembly, the tooling plate and coil assembly is wrapped with a silicone sheet, and the entire assembly is placed in a vacuum bag and is cured in an autoclave while vacuum is drawn on the bag. The bagging and autoclave curing facilitate production of a coil substantially free of voids in the dielectric material isolating adjacent coil turns from each other.
After curing of the coil dielectric and removal of the tooling plates, fastener holes are drilled in the coil leads and mechanical fasteners are used to connect the coil leads to a pair of conductor wires. The coil leads and wires are laid into the recesses in the end faces of the inner hub and outer body and are potted in place. A rigid plate is then secured across the end faces to cover the potted-in coil leads and conductor wires to complete the coil assembly.